1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling a high voltage in an electrophotographic image forming apparatus such as a laser printer, and more particularly, to an apparatus for and a method of controlling a high voltage by appropriately adjusting a sequence and a value of the high voltage, which includes an alternating current voltage superimposed on a direct current voltage, to be applied to a developer for printing a high quality image by preventing the forming of an image artifact such as a band.
2. Description of Related Art
Printers, copiers, facsimiles, or multi-function printers include an image forming apparatus to print an image. Generally, the image forming apparatus prints an image through charging, exposing, developing, transferring, and fusing processes.
FIG. 1 is a schematic view of a conventional image forming apparatus. Referring to FIG. 1, the image forming apparatus includes a charging roller 20, an organic photoconductive (OPC) drum 30, a transfer roller 40, a developer 50 including a developing roller 52 and a high voltage supply roller 56 for supplying a high voltage, a fuser 70, and a laser scanning unit (LSU) 80 to print image data on a sheet of paper.
The operation of the image forming apparatus will now be described. When a charging voltage is applied to the charging roller 20, the charged charging roller 20 rotates, thereby evenly charging an organic photoconductor formed on an outer circumferential surface of the OPC drum 30. Then, light emitted from the LSU 80 forms a latent electrostatic image on the surface of the charged OPC drum 30. A high voltage is then applied to the high voltage supply roller 56 and a lower voltage is applied to the developing roller 52. This difference in the levels of the applied voltages results in a difference between a potential of the supplying roller 56 and that of the developing roller 52, forcing toner with negative electric charge components to move from the roller for supplying a high voltage 56 to the developing roller 52. The toner supplied to the developing roller 52 is applied to the latent electrostatic image formed on the OPC drum 30, thereby forming a visible image. The high-voltage transfer roller 40 transfers the visible image onto a sheet of paper being moved. The visible image transferred onto the sheet of paper is fused on the sheet of paper with the high heat and pressure of the fuser 70. This is the end of the printing process.
In the printing process, the charging, developing, and transferring processes involve applying a high voltage of hundreds through thousands of volts to rollers. Image quality is determined by how much the high voltage applied to the rollers is controlled.
A conventional apparatus for controlling a high voltage applied to a developer is disclosed in U.S. Pat. No. 5,627,722. This apparatus uses a method of applying a high voltage with an alternating current (AC) voltage superimposed on a direct current (DC) voltage to a developer of an image forming apparatus. FIG. 2 is a block diagram of the conventional apparatus for controlling a high voltage of the '722 patent and FIG. 3 is a waveform diagram of a voltage output from the conventional apparatus for controlling the high voltage of the '722 patent.
Referring to FIG. 2, a controlling unit 100 included in the image forming apparatus controls a high AC voltage output unit 110 and a high DC voltage output unit 130 to control a high voltage with the AC voltage superimposed on the DC voltage, which is applied to a developer 150. To control the high voltage, the controlling unit 100 includes an AC peak-to-peak controller 102, an AC duty controller 104, an AC enable signal generator 106, and a DC pulse width modulation (PWM) controller 108. The controlling unit 100 may be a microprocessor using a control program. In this case, the AC peak-to-peak controller 102, the AC duty controller 104, the AC enable signal generator 106, and the DC PWM controller 108 may be terminals included in the microprocessor.
The AC peak-to-peak controller 102 generates an AC_Vp-p signal for controlling a peak-to-peak voltage of an AC voltage ACVpp. The AC voltage ACVpp is generally controlled using a PWM and ranges from 1 KV to 2.8 KV. The AC duty controller 104 generates an AC_Vduty signal for controlling a duty of the AC voltage ACVpp, that is, the on/off time of an AC voltage frequency, output from the high AC output unit 110. The AC duty controller 104 controls the duty of the high AC voltage ACVpp at 20-50% based on a negative voltage by using the PWM. The AC enable signal generator 106 generates an AC_Enable signal for enabling the high AC voltage output unit 110 to apply the high AC voltage ACVpp to the developer 150. If the AC_Enable signal is not generated, even though the AC_Vp-p signal and the AC_Vduty signal are generated, the AC voltage ACVpp is not applied to the developer 150.
The DC PWM controller 108 generates a DC_PWM signal for controlling the DC voltage DCV output from the high DC voltage output unit 130 using the PWM. The DC voltage DCV output from the high DC output unit 130 controlled by the DC PWM controller 108 ranges from 20V to 1000V and is applied to the developer 150.
The controlling unit 100 controls the high AC voltage output unit 110 and the high DC voltage output unit 130 such that they apply a high voltage with the AC voltage ACVpp superimposed on the DC voltage DCV to the developer 150. Here, the DC voltage DCV is first applied to the developer 150. Then, after a predetermined period of time, the controlling unit 100 sequentially transmits the AC_Vp-p signal, the AC_Vduty signal, and the AC_Enable signal to the high AC output unit 10, which then applies the AC voltage ACVpp to the developer 150.
This process will now be described in more detail with concurrent reference to FIGS. 1 and 3. As described with reference to FIG. 1, in a conventional printing sequence, a charging voltage is applied to the OPC drum 30. Then, a high voltage with the AC voltage ACVpp superimposed on the DC voltage DCV is applied to the developer 150. First, the DC voltage DCV is applied to the developer 150 at a duty (approximately 35% with 300V) required for printing. After the DC voltage DCV is applied to the developer 150, the AC voltage ACVpp is applied to the developer 150. Ultimately, the high voltage with the AC voltage ACVpp superimposed on the DC voltage DCV is applied to the developer 150. In response to the high voltage, the toner with negative electric charge components is moved to the developer 150 and applied to the latent electrostatic image formed on the surface of the OPC drum 30, thereby forming a visible image.
Values of the high voltage applied to the developer 150 are as follows. An initial duty of the AC_Vp-p signal generated by the controlling unit 100 to enable the high AC voltage output unit 110 to output the AC voltage ACVpp is approximately 17-25%, which is a set value required for printing. The AC voltage ACVpp output from the high AC voltage output unit 110 ranges from approximately 2000V to 2400V. When the AC_Vp-p signal is transmitted to the high AC voltage output unit 110, the AC_Vduty signal is also transmitted to the high AC voltage output unit 110. Here, a duty of the AC_Vduty signal is the same as a duty required for printing, that is, approximately 30-40%. Approximately 20 msec later after the AC_Vp-p signal and the AC_Vduty signal start to be transmitted to the high AC voltage output unit 110, the AC_Enable signal is turned on and transmitted to the high AC voltage output unit 110. When the AC_Enable signal is turned on, the AC voltage ACVpp is output from the high AC voltage output unit 110 and superimposed on the DC voltage DCV that is already being output from the high AC voltage output unit 110. Then, the superimposed AC voltage ACVpp and the DC voltage DCV are applied to the developer 150.
While the DC voltage DCV is being applied to the developer 150, if the AC voltage ACVpp is turned on AC ON, an undershoot and/or high voltage noise may be generated as illustrated in FIG. 3. As a result, an image artifact such as a band may be formed in a final image.